The preparation of silica sols that contain non-aggregated spherical particles is well known. A means of preparing such silica sols includes using an acidic, active solution of silicate (sometimes referred to as silicic acid) and grows particles in an alkaline water based media at elevated temperatures. Typically the resulting solutions are low in concentration and are normally further concentrated before sale. An alternative route is to concentrate via evaporation while the particles are growing (i.e., during the reaction).
Typical means to grow particles are via a batch basis or with a continuous system. Batch basis techniques normally make narrow distribution particles. Continuous systems typically make broad distribution particles. To have good particle size control, continuous systems are complex and employ multiple overflow reactors. Besides being capital intensive, such continuous reactors do not have good control over particle size distribution and are limited in average particle size (e.g., U.S. Pat. No. 5,964,693 utilizes five overflow reactors and discloses particles of 27 to 72 nm). Another disadvantage of these continuous systems is that while very small particles can be reduced they cannot be totally eliminated.
There thus exists an industry need to eliminate the disadvantages of continuous reaction systems. Particularly, there is a need to economically produce silica particles having a controlled minimum particle size and wide distribution curve.